|MadSci Network: Earth Sciences|
Good question. The term "Ground Zero" comes from the military world where it refers to the exact location where a bomb hits. Around that spot are drawn concentric circles labeled G+5, G+10, G+50... and so on showing how far out from the impact point the circle is. Since it is the impact point for the bomb, it is, of course, the point of highest damage. That's how the term came to be used with earthquakes - it is the point of highest damage. The epicenter of an earthquake is the spot on the surface of the Earth directly over the source of the quake. Since it is closest to the actual source (which may be kilometers deep in the Earth), the epicenter is often (but not always) the point of greatest damage - thus Ground Zero
Try the links in the MadSci Library for more information on Earth Sciences.